A search for mitotic recombination in mammalian cells will be carried out using a cultured Chinese hamster ovary cell line. Cells carrying mutations for the two synthetic genes governing the enzymes hypoxanthine phosphoribosyl transferase and glucose-6-phosphate dehydrogenase will be fused to construct hybrids containing various combinations of mutant and wild type alleles. Segregants of these hybrids will be selected that exhibit recombinant phenotypes. The formation of new linkage relationships will be established by further hybridization and segregation studies. Treatments known to promote recombination in lower organisms will be tested for their effectiveness in mammalian cells. Finally, an attempt will be made to map the locus for hypoxanthine phosphoribosyl transferase by intragenic mitotic recombination. The objectives of the proposed research are to determine if mitotic recombination is a source of genetic variation in somatic mammalian cells, and if so, to exploit this process to develop methods for fine structure genetic mapping.